A year after a Globe investigation found restaurants and stores across Massachusetts were routinely selling cheaper, lower quality fish than they promised customers, a new round of DNA testing shows the vast majority of those venues are still mislabeling seafood. Of the 76 seafood samples tested by the Globe, 76 percent were mislabeled for the second year in a row. The Globe focused most of its testing on certain species that have been identified by regulators as more likely to be mislabeled.